This invention relates to a carriage assembly capable of positioning a recording head at any desired position with respect to the surface of a disc.
FIG. 1 illustrates, in perspective, an example of a conventional carriage assembly for use in an optical disc apparatus.
Such a conventional carriage assembly comprises a frame 1, an objective lens 2 for projecting a light beam toward an optical disc 3 which rotates about a shaft 3a. A carriage 4 is movably supported within the frame 1 and comprises a translating member 4a of substantially rectangular cross-section having a data converter unit (not shown) incorporated therein, and a pickup assembly 4b mounted on one end portion of the translating member 4a. The pickup assembly 4b also carries thereon the objective lens 2. Further included in the assembly is a radial guide arrangement 7 for permitting the carriage 4 to move in a radial direction relative to the disc 3. the radial guide arrangement 7 comprises linear bearings 8 mounted on both sides of the translating member 4a of the carriage 4 and a pair of guide rails 9 secured to the frame 1 in a parallel relationship with respect to each other and in the radial direction relative to the disc 3. the carriage assembly further comprises a radial drive arangement 10 including an electromagnetic coil 11, which has coil windings 11a and is secured to the translating member 4a of the carriage 4, and a pair of magnetic circuits 12. Each of the magnetic circuits 12 comprises an outer stator yoke 13 of a magnetic material having a substantially U-shaped horizontal cross section, a permanent magnet 14 disposed in an inner side of the bight portion of the U-shpaed yoke 13 and a center pole piece 15 connected to the ends of the leg portions of the U-shaped yoke 13, through which a magnetic flux flows. The center pole piece 15 is electromagnetically coupled with the electromagnetic coil 11. The optical lens 2, the carriage 4, the linear bearings 8 and the electromagnetic coil 11 together constitute a moving unit. the guide rails 9 and the magnetic circuits 12 together constitute a stationary unit.
When the electromagnetic coil 11 is energized in accordance with a command from an external control unit (not shown), a magnetic field is generated around the coil windings 11a and the thus generated magnetic field generates a driving force in cooperation with the magnetic flux flowing through the respective center pole pieces 15 as defined by Fleming's rule, whereby the carriage 4 together with the objective lens 2 are moved in the direction indicated by an arrow A or A' shown in FIG. 1, i.e. in the radial direction relative to the disc 3. the objective lens 2 mounted on the pickup assembly 4b is arranged so as to be movable in the radial direction and/or in the vertical direction relative to the disc 3 to constantly form a read spot of the light beam on the disc surface. For this purpose, the pickup assembly 4b includes an adjusting device (not shown) mounted therein for adjusting the focus of the objective lens 2 at any desired position on the disc surface by moving it in the radial direction to the disc 3 as well as in the perpendicular direction to the disc surface.
When a recording or reproducing function is to be performed, first the objective lens 2 is approximately positioned over a desired area on the disc 3 by regulating the current supplied to the electromagnetic coil 11. Once the lens 2 is positioned over the desired area of the disc, the current supplied to the electromagnetic coil 11 is cut off, whereby the driving force ceases. Then, the objective lens 2 is moved in the radial direction and/or perpendicular direction to the disc 3 by means of the adjusting device disposed in the pickup assembly 4b, so that the light beam generated by the data converter unit (not shown) located in the carriage 4 and projected through the lens 2 is precisely positioned on a desired position on the disc surface.
The thus-constructed conventional carriage assembly has problems in that, since the electromagnetic coil 11 is disposed on one side of the pickup assembly 4b and with a certain distance from the pickup assembly 4b as shown in FIG. 1, the physical dimension in the travelling direction of the moving unit is large, whereby it becomes difficult for the carriage assembly to be made compact. Further, since the side portions of the electromagnetic coil 11 may be deformed in the travelling direction as indicated by an arrow A or A' in FIG. 1 due to the driving force, it is difficult to stop the carriage 4 at a desired position with precision, and therefore it may not be possible to precisely position the light beam projected through the objective lens 2 at a desired position on the disc surface. In order to increase the precision of movement of the moving unit, the rigidity of the electromagnetic coil 11 must be increased in the travelling direction of the electromagnetic coil 11. For this purpose, it is necessary to increase the thickness of the electromagnetic coil 11, such that the electromagnetic coil 11 is not deformed in the travelling direction thereof.